CVE-2026-7838 UltraVNC Viewer Heap Buffer Overflow Vulnerability
UltraVNC viewer through 1.8.2.2 contains an integer overflow leading to a heap buffer overflow in the RFB protocol failure-response parsing path. In vncviewer/ClientConnection.cpp, the 4-byte network-supplied reasonLen field (type CARD32) is passed as reasonLen+1 to CheckBufferSize(). Because both operands are unsigned 32-bit, a reasonLen of 0xFFFFFFFF overflows to 0, causing CheckBufferSize to allocate only 256 bytes. The subsequent ReadString(m_netbuf, reasonLen) call then performs ReadExact for the original 4 GiB length into that 256-byte heap buffer. This overflow is reachable via rfbConnFailed (auth-scheme negotiation) and rfbVncAuthFailed (post-handshake) message types without successful authentication. A malicious VNC server, or any man-in-the-middle on the RFB stream, can trigger this condition when the victim viewer connects, potentially resulting in remote code execution as the user running the viewer. The crash was confirmed with AddressSanitizer on a portable reproduction harness (heap-buffer-overflow WRITE at offset 256).
Quick answer
UltraVNC Viewer should be reviewed and updated if it matches the affected versions. The recommended fix is to apply the vendor-supported patched version or the mitigation steps below, then retest the public website with Fixnx.
Who is affected
Affected versions
- <= 1.8.2.2
Fixed versions
- No vendor fix published yet
How to fix it
UltraVNC Viewer is affected and no public fixed version is listed in the local advisory data yet. Reduce access, disable risky features where possible, and apply the vendor fix when it is released.
- Check where UltraVNC Viewer is installed and whether it is reachable from the internet.
- Limit access to trusted users, trusted networks, or a VPN until a vendor fix is available.
- Disable the affected feature or module if the advisory points to a specific upload, login, admin, chat, or network service path.
- Watch the vendor or advisory source and apply the first supported fix as soon as it is released.
- Review logs for strange access, failed attempts, crashes, file uploads, or unexpected account changes.
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Verify the fix
- Confirm the installed version is no longer in the affected range.
- Retest the affected page, API, upload, login, chat, container, or network flow.
- Check logs after the change for blocked attempts, crashes, or errors.
- Run a fresh Fixnx scan and save the report.
Related categories
Trusted references
FAQ
What is affected by CVE-2026-7838?
UltraVNC Viewer versions listed as affected should be reviewed: <= 1.8.2.2.
What should I fix first?
Start with internet-facing sites, admin panels, login flows, plugins, themes, modules, packages, and systems that process user-controlled input or sensitive data.
How do I confirm the fix worked?
Apply the patched version or mitigation, clear caches where relevant, retest the affected workflow, and run a new Fixnx scan to verify public website exposure signals.
How are Fixnx security risk categories chosen?
Fixnx keeps one canonical risk page and assigns only broad, relevant categories such as ecosystem, technology area, or vulnerability class.